1. Field of the Invention
The present invention relates to a heating furnace and, more particularly, to a continuous heating furnace with a local heating device employed therein for uniformly heating workpieces each having a bar like configuration with an upset portion at least at one of its opposite ends.
2. Description of the Prior Art
In heating bar like articles such as pipes, tubes, rods and the like (generally referred to as pipes), for heat-treating purposes or for bringing them to required temperatures for subsequent operations, it is very important that the workpieces be uniformly heated throughout. For this purpose, a furnace provided with a conveying apparatus is known, such as disclosed in Charles MacGregor's U.S. Pat. No. 2,858,122 issued Oct. 28, 1958, which comprises: a furnace having an entry port, from which the pipes and the like are inserted in its axial direction one after another with a predetermined interval; a conveying apparatus provided in the furnace for laterally conveying the pipes in a direction perpendicular to said axial direction; and an exit portion, from which the heated pipes are expelled. According to this prior art, the pipes are heated during their lateral conveyance in the furnace and, therefore, the furnace has a predetermined size capable of heating the pipes to the required temperatures.
In the case where the pipes have an upset portion, where the diameter is increased, at its one end or at its opposite ends, additional heat must be added to such an upset portion to bring the whole pipe to the required temperatures. To accomplish this, one may propose to enlarge the size of the furnace to elongate the distance of lateral conveyance of the pipe, thus keeping the pipe in the furnace for a long time. This arrangement however, results in a bulky size of furnace and in an increase of occurrence of scaling of the pipes. Furthermore, the fuel necessary to heat up the furnace would increase greatly when compared with a small increase (several %) in weight in the pipe.
To overcome such drawbacks, there has been proposed an improved furnace having a pre-heating device provided externally to the furnace for pre-heating the upset portions before the pipes enter into the furnace, whereby the upset portions can be heated in the furnace up to the required temperatures in a period of time which is about the same time required for heating a body portion of the pipe (a portion other than the upset portions).
The employment of such a pre-heating device, which is provided with a burner or inductor for effecting the heating, results not only in an increase of the initial cost and running cost, but also in an increase of heating time. Furthermore, it results in an increase of the entire size of the furnace because of such a pre-heating device provided externally.
Another improved furnace has been proposed and is disclosed in Japanese Laid-Open Patent Application No. 55-94442 issued July 17, 1980, and both of the present inventors are co-inventors of the improved furnace disclosed therein. According to this improved furnace, a blower is provided on one or both sides of a path of the upset portion for blowing high temperature gas towards the path of the upset portion. This improved furnace, however, has a drawback in effective and efficient utilization of the blown out high temperature gas.